As sizes of transistors decrease, forming a thin film such as an insulating film which constitutes a sidewall spacer (SWS) of a gate electrode may require low temperature for film forming, increased resistance to hydrogen fluoride (HF), and a small dielectric constant. To this end, it has been studied to use, as an insulating film, a silicon boronitride (SiBN) film (i.e., a silicon nitride (SiN) film to which boron is added) or a silicon borocarbonitride (SiBCN) film (i.e., a silicon nitride (SiN) film to which carbon as well as boron is added).
Such an insulating film is often formed by an alternating supply method that alternately supplies several kinds of process gases because a high step coverage is required. For example, using a silicon (Si)-containing gas as a source gas, boron trichloride (BCl3) gas or diborane (B2H6) gas as a boron source, ammonia (NH3) gas as a nitrogen source, and ethylene (C2H4) gas or propylene (C3H6) gas as a carbon source, a SiBCN film can be formed on a substrate by performing a cycle, which sequentially supplies the above process gases to the substrate, a predetermined number of times. However, this method that separately supplies the boron source, the nitrogen source, and the carbon source leads to a long time for a single cycle, which results in a low productivity of a film forming process.